Let’s describe my current dual boot setup between postmarketOS and android on my shift6mq shift-phone.

• Slot A: postmarketOS /boot with rootfs on SD card (mmcblk0)
• Slot B: android (shiftOS-G 5)

Installation:

I’ve followed the instructions at https://wiki.postmarketos.org/wiki/SHIFT_SHIFT6mq_(shift-axolotl).

Enter Fastbootd and ensure it is working as expected:

$ # Set "a" as active slot and reboot to fastbootd
$ fastboot --set-active=a
$ fastboot reboot fastboot
$ # Verify current slot is "a"
$ fastboot getvar current-slot

After you verified fastbootd works in the slot you have selected, you can install postmarketOS:

$ # Flash rootfs to system_a partition
$ pmbootstrap flasher flash_rootfs --partition system_a
$ # Flash kernel to boot
$ pmbootstrap flasher flash_kernel --partition boot_a
$ # Reboot to bootloader, as erasing dtbo is not working in fastbootd
$ fastboot reboot bootloader
$ # Erase dtbo_a partition, as it conflicts with our mainline kernel
$ # Note: this operation takes some time to complete
$ fastboot erase dtbo_a
$ # Reboot into postmarketOS
$ fastboot reboot

After rebooting, the shift6mq boots into postmarketOS, but the available space is very limited. So, I installed postmarketOS for a second time on a SD card with:

$ pmbootstrap init
$ pmbootstrap install --sdcard=/dev/mmcblk1

I’ve put that sdcard in my shift6mq and booted into postmarketOS and altered /etc/fstab so it points to the /rootfs partition on the SD card instead of the internal storage.

# <file system> <mount point> <type> <options> <dump> <pass>

# / on mmcblk0p2 (sdcard)
UUID=4540a2ee-6dc5-4294-8b24-a479b95836df / ext4 defaults 0 0

# /boot via boot_a
#mmcblk0p1: UUID=cb3d185d-4eb3-40b6-97e7-b6844e9493f7 /boot ext2 nodev,nosuid,noexec 0 0
UUID=82b9205c-2815-42ea-890a-aa15d1e50633 /boot ext2 nodev,nosuid,noexec 0 0

Then I ran the command mkinitfs. After rebooting, the shift6mq boots, but with / on the SD card.

s6sxmo:~$ duf -hide special
╭─────────────────────────────────────────────────────────────────────╮
│ 1 local device                                                      │
├────────────┬────────┬───────┬───────┬───────┬──────┬────────────────┤
│ MOUNTED ON │   SIZE │  USED │ AVAIL │  USE% │ TYPE │ FILESYSTEM     │
├────────────┼────────┼───────┼───────┼───────┼──────┼────────────────┤
│ /          │ 116.4G │ 20.3G │ 90.1G │ 17.5% │ ext4 │ /dev/mmcblk0p2 │
╰────────────┴────────┴───────┴───────┴───────┴──────┴────────────────╯

Problem:

Nice, lot’s of free space and able to switch to slot B to start android. This seems perfect… but is it? No!

Strange, /boot isn’t mounted anymore and I’m not able to mount it again. Because of this mkinitfs will fail. Doing an apk upgrade where a new kernel is installed will fail. So, everytime there is a kernel upgrade I have to do:

$ pmbootstrap init
$ pmbootstrap install
$ pmbootstrap flasher flash_rootfs --partition system_a
$ pmbootstrap flasher flash_kernel --partition boot_a
$ vim /etc/fstab
$ mkinitfs

Not tested:

This isn’t tested very well. Lately I’m able to break things a lot, so this will probably not be different ;)